The immunological mechanism involved in the pathogenesis of rheumatic fever and rheumatic heart disease will be studied. The parameters to be investigated include: 1. The presence of antigenic similarity between human valvular tissue and streptococcal cell wall constituents. 2. The role of humoral and cell mediated immunity in the production of tissue damage and in determining susceptibility to rheumatic fever. 3. Differences in the antigen processing mechanisms of phagocytic cells, primarily in the enzymes responsible for the degradation of the streptococcal components, as a determinant of susceptibility to rheumatic fever. In addition, a search for specific biological characteristics of the group A streptococcus that might explain the pathogenic characteristic of this organism will be carried out.